1. Field of the Invention
The present invention relates to a method for fabricating optical components. More particularly, the present invention relates to a method for forming microlenses of different curvatures, and to a fabricating process of a solid-state image sensor which utilizes the method so that the sensitivities to different color lights can be optimized respectively.
2. Description of the Related Art
A solid-state image sensor, such as a CCD image sensor or CMOS image sensor (CIS), is essentially composed of red, green and blue pixel sensors that are respectively equipped with red, green and blue filters. To increase the quantum yield of incident light for improving the sensitivity, a microlens can be formed over each color filter to focus the incident light, as described in U.S. Pat. No. 6,379,992, for example.
Referring to FIG. 1A, in a typical microlens process of a CIS device, a substrate 100 is provided, on which a passivation layer 110 for protecting the circuit of the sensor, blue, green and red filters 120, 130 and 140, and a planarization layer 150 have been formed. A transparent photosensitive layer 160 is formed on the planarization layer 150 and then patterned into islands 160a of the same thickness, wherein each island 160a is form over one color filter.
Referring to FIG. 1B, the islands 160a are heated and softened such that the surface tension makes their surfaces spherical, thus forming multiple microlenses 160b, which have the same curvatures because all of the islands 160a have the same thickness. Hence, the focal lengths of the three microlenses 160b over the blue, green and red filters 120-140 are the same.
However, as shown in FIG. 1B, because respective maximal absorption regions 122, 132 and 142 of blue, green and red lights 124, 134 and 144 are different in depth in the substrate 100, the absorptions of the blue, green and red lights cannot be optimized respectively with the microlenses 160b of the same curvature that have a single focal length only. Hence, the sensitivities of the image sensor to different color lights are different to cause chromatic deviation of the recorded images.
To solve the above problem, for example, the method disclosed in U.S. Pat. No. 5,592,223 can be used, in which the sensitivities to different color lights are equalized by varying respective areas and/or curvatures of the microlenses formed over different color filters. Besides, the method provided in U.S. Pat. No. 6,643,386 equalizes the sensitivities to different color lights by altering respective colors or shapes of the microlenses over different color filters, wherein the shapes include areas and curvatures.
However, when the sensitivities are equalized by forming microlenses of different areas or colors, a color light of higher sensitivity is made to have a lowered intensity incident to the corresponding sensor diode. Therefore, the overall sensitivity of the CIS device cannot be improved. In addition, forming microlenses of different colors needs more than one lithography processes, so that the fabrication is tedious.
On the other hand, when the sensitivities are equalized by forming microlenses of different curvatures, the sensitivities to different color lights can be respectively optimized by adjusting respective curvatures to obtain different focal lengths. However, this microlens process is very tedious because three lithography processes are conducted to form photoresist islands of three thicknesses for making the three curvatures.